Tomography is explained in The Fundamentals of Radiography (12th Edition) by Eastman Kodak Company, Health Sciences Markets Division, Rochester, N.Y. 1980.
Tomography is a method of reducing the "clutter" of overlying or underlying structures which obscure desired information in a radiograph. It is a technique which provides an image of any selected plane through the body, while blurring out images of structures that lie above or below that plane.
Typically, a tomogram is obtained by a special mechanism that moves the x-ray tube and film in opposite directions simultaneously. FIG. 1 shows one such method.
FIG. 1 is a schematic diagram illustrating a traditional mechanically sweeping tomographic system. The tomographic system has an x-ray tube (3) and an image receptor (11) for receiving the x-ray radiation. The image receptor (11) contains a film (1) which is supported on a Bucky tray (8). During exposure of a patient's body (5), the film (1) and the x-ray tube (3) are moved horizontally in opposite directions as indicated by the arrows (2) and (4). This movement is typically achieved by rotating a lever (not shown) attached to the x-ray tube (3) and the Bucky tray (8) which carries the film (1).
The pivot point (6) or fulcrum about which the lever rotates is adjustable so that any desired horizontal layer within the body (5) can be selected for imaging. The horizontal plane in the body (5) which contains the pivot point (6), about which the x-ray tube (3) and the film (1) move, remains in focus. This is shown in FIG. 1 as the focal plane (7). Structures in this focal plane (7) are not affected by the motion of the x-ray tube (3) and the film (1) in opposite directions (2) and (4). These structures appear as they would in a stationary radiograph. The images of structures above and below the fulcrum or focal plane (7) are blurred by the motion of the x-ray tube (3) and Bucky tray (8) which carries the film (1). The greater their distance from the focal plane (7), the greater the blurring.
There is a region on each side of the focal plane (7) in which the blurring due to the motion (2, 4) does not exceed that arising from other geometric factors and light diffusing in the image receptor (11). Therefore, the thickness of the layer, within the body (5), which can be imaged satisfactorily depends on the characteristics of the recording system, the body (5) involved and the subjective needs of the radiologist. The thickness of this "in-focus" slice depends on the angle (9) through which the x-ray (3) tube moves and the distance (10) of the focal plane (7) from the film (1). The larger the angle of swing (9) and the farther the focal plane (7) is from the film (1), the thinner the cut or section which will be in focus.
In short, the quality of a tomographic system and its ability to focus on a particular layer of a body (5), while filtering unwanted images is a function of various items. It is a function of the accuracy of moving the film (1) and x-ray tube (3) synchronously in opposite directions (2, 4), the accuracy of positioning the fulcrum or pivot point (6) and the accuracy of the interval resolution in which the focal plane distance (10) can be adjusted. The quality of a tomographic system is also a function of the accurate reproducibility of that distance (10), slice thickness of body (5), and x-ray exposure timing.
Throughout the tomographic sweep, two things are essential to image quality. First, maintaining the mechanical center-to-center alignment of the x-ray source or tube (3) to the image receptor (11) during x-ray tube (3) angulation. Second, ensuring the smoothness of the longitudinal travel of the x-ray tube (3) and the image receptor (11).
The accuracy of tomography is dependent upon the angle of swing (9) and centering of the tomographic sweep at the pivot point (6). This accuracy is also dependent on the x-ray exposure timing. Yet, the weight of the tube assembly (not shown) enclosing the x-ray tube (3) and the remaining structure containing the image receptor (11), the Bucky tray (8) and the film (1), can be several hundred to as much as a thousand pounds. This heavy weight makes movement in a controlled and precise manner difficult. Furthermore, for accurate tomography, the time of the sweep has to be correlated to the anticipated exposure time of the film (1). Or, conversely, the film exposure time has to be correlated to the projected sweep time.
Because the film exposure is dependent on the patient and the patient's body density, automatic exposure timing is used in most radiographic applications. Such an automatic exposure control operates as follows. The power settings of the x-ray tube (3), i.e., the kV and mA settings, are selected and the x-ray tube (3) energized. When the desired film density is achieved, an ionization chamber (12) or similar pick up, terminates the exposure.
In one traditional automatic exposure control system, shown in FIG. 2, exposure termination occurs as follows. An ionization chamber (12) generates an ionization current (13) from the x-ray energy passing through the patient's body (5). The ionization chamber (12) is connected to the Bucky tray (8). The ionization current (13) is inputted to an integrator (14). The integrator (14) integrates the ionization current (13). The integrated ionization current (15) is also referred to as a voltage ramp signal (15) shown in FIG. 3.
FIG. 3 is a plot of voltage versus time. The vertical axis represents voltage (16) and the horizontal axis represents time (17). The slope of the ramp signal (15) is proportional to the rate that x-ray energy is passing through the patient. The system sets a reference voltage (18), which is the required voltage to achieve the desired film density. The reference voltage (18) is a maximum threshold voltage, above which exposure is terminated
FIG. 3 shows the exposure time (19) on the horizontal time axis (17). The time it takes for the integrated ionization current or voltage ramp signal (15) to cross the reference voltage (18) is the exposure time (19). The optimal time of exposure is dependent on the selected kV and mA settings, and the patient's body size.
As shown if FIG. 2, a cut-off circuit (28) generates a cutoff signal (29) when the amplitude of the ramp signal (15) reaches a value equal to the reference voltage (18). When the amplitude of the ramp signal (15) equals this reference voltage (18), the generated cut-off signal (29) cuts off the power of the x-ray tube and terminates the exposure.
Tomographic accuracy depends on proper optical density on the film (1) and the proper sweep time for the motion in opposite direction (2, 4) of the film (1) and the x-ray tube (3). Achieving the required optical density on the film (1) and the required sweep time for tomographic accuracy is almost impossible to do at the same time.
It is an object of the present invention to provide an automatic exposure control system for tomographic applications which correlates exposure of the film to the tomographic sweep time.